In disc brakes a brake lining being arranged at the end of a thrust element is forced against a brake disc, wherein the thrust element, which is part of a brake actuating mechanism, is guided in a housing of a brake caliper. In this connection the thrust element is actuated generally via several interposed components of, among others, an adjustment mechanism by means of a lever, which lever is supported in the rear section of the housing of the brake caliper.
Accordingly, at brake actuation the thrust element performs an actuation stroke which is directed towards the brake disc. Upon release of the brake, when no more braking force is introduced, the thrust element performs a release stroke in the reverse direction, which normally is encouraged by a spring-like return mechanism. Both actuation stroke and release stroke are substantially directed parallel with the brake disc axis, since the thrust element as being part of the brake actuating mechanism is axially displaceably guided in the housing of the brake caliper by means of at least one axial guiding element.
Since, taking into consideration the thickness of the brake linings, the distances of the motion during brake actuation and brake release are known by the predefined dimensions for each embodiment of a brake actuating mechanism, a too large stroke could be used as an indicator for any kind of malfunction of the brake actuating mechanism. Reasons for a too large stroke might be due, for example, to a defect adjustment mechanism, to a too large wear of the brake linings or similar.
Likewise, a too short stroke might indicate a malfunction of the return mechanism, of the actuating cylinder or similar.
Therefore, for reasons of functional safety of the brake system and thus in the broader sense for road safety reasons it is absolutely necessary to monitor and to accordingly detect the actuation stroke and/or the release stroke in any form, since an excessive stroke either speaks for a soon to be expect breakdown of the brake actuating mechanism or indicates the necessity to replace the brake linings soon, whereas a too short stroke speaks for a soon to be expect breakdown of the return mechanism or the brake cylinder.
At present three different measurement principles are applied in the prior art.
For instance, several indicator devices provide an alarm signal to the driver or to a superior control system as soon as the stroke exceeds a predetermined length. Such indicators often are based on switch mechanisms which are controlled by magnetic or mechanic means. Although such indicator devices generally prove to be rather cheap, they however require a highly precise positioning and adjustment during their initial assembly within or at the periphery of the brake caliper on the one hand and during a later maintenance of the detection mechanisms on the other, which in turn will again increase the overall costs.
A further principle refers to the detection of the stroke which is actually present. Sensors, which are used for such a quantitative detection method, provide signals which reflect the linear motion of the brake actuating mechanism with a relatively high accuracy. However, such sensors prove to be extremely costly, in particular if only an excessive stroke shall be determined.
Embodiments of prior art monitoring devices of such kind are e.g. known from U.S. Pat. No. 5,433,296 or U.S. Pat. No. 7,398,141 B2.
A third principle refers to the measurement of the actually existing condition of the brake linings. Sensors for that purpose mainly are applied in connection with sophisticated brake control systems which are used in dynamic vehicle control and assistance systems in order to compensate the wear of the brake linings among different single brakes of the vehicle. Such sensors also prove to be too expensive if their single purpose shall be the detection of the stroke.
Configurations of prior art detection devices of this type can be found e.g. in U.S. Pat. No. 5,339,069, US 2009/0050418 A1 or DE 195 34 854 A1.
For most safety purposes, and even sometimes supported by the legislative body, it is entirely sufficient to monitor the brake stroke with a rather determined but just approximate accuracy, which does not reproduce the exact, actually existing value of the stroke; in other words, it can be completely sufficient to only provide a qualitative rather than a quantitative indication of the actually present stroke.